<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        #small{
            width: 300px;
            height: 300px;
        }
        #small>img{
            width: 300px;
            height: 300px;
        }
        #slider{
            width: 140px;
            height: 140px;
            position: absolute;
            top: 0;
            background-color: rgba(208, 255, 0, 0.306);
            display: none;
            }
        #big{
            width: 300px;
            height: 300px;
            position: absolute;
            top: 0;
            left: 350px;
            overflow: hidden;
            display: none;
        }
    </style>
</head>
<body>
    <div id="small">
        <img src="fdj.jpg" alt="">
        <div id="slider">
        </div>
        <div id="big">
            <img src="fdj.jpg" alt="">
        </div>
    <script>
        var small = document.getElementById('small');
        var big = document.getElementById('big');
        var slider = document.getElementById("slider");
        
        var bigImg = document.getElementsByTagName("img")[1];
        small.onmousemove = function(e){
            var even = e || event;
            var x = even.clientX - small.offsetLeft - slider.offsetWidth / 2;
            var y = even.clientY - small.offsetTop - slider.offsetHeight / 2;
            var maxX = small.clientWidth - slider.clientWidth;
            var maxY = small.clientHeight - slider.clientHeight;
            if(x<0){
                x=0;
            }
            if(x>maxY){
                x=maxX;
            }
            if(y<0){
                y=0;
            }
            if(y>maxY){
                y=maxY;
            }
            slider.style.top=y+'px';
            slider.style.left=x+'px';
            big.scrollLeft=x/maxX*(bigImg.clientWidth-big.clientWidth);
            big.scrollTop=y/maxY*(bigImg.clientHeight-big.clientHeight);
        }
        small.onmouseenter=function(){
            slider.style.display='block';
            big.style.display='block';
        }
        small.onmouseleave=function(){
            slider.style.display='none';
        }
    </script>
</body>
</html>